"I presume that inside oVirt the disk itself is marked as bootable, correct?"
yes.
"Which type has been assigned to it?"
Type. What do you mean?
"In the original exported VM do you remember which type of disk/controller you used for the boot disk?"
VirtIO or VirtIO SCSI
"In case you can edit it (with VM powered down) and try both virtio and virtio-scsi as interface types."
Already done! No success...
The others linux machines are imported with success.
https://lists.ovirt.org/archives/list/users@ovirt.org/message/7ZP62GNJOOVG25IPQR42YKJ7EDQZCL3Q/